The present invention relates to a novel carbon dioxide fixation cycle and a method for fixing carbon dioxide using the same.
Modern industry has the structure in which most source materials and chemical energy are derived from fossil fuels. However, fossil fuel reserves are finite, and the extensive use of fossil fuels causes serious environmental problems such as the increase in carbon dioxide concentration in air. Elevated carbon dioxide concentration in air has been pointed out as a main cause of global warming. To solve such problems, various types of research and development are being conducted to chemically collect or biologically fix carbon dioxide, which would result in reduction of carbon dioxide of atmosphere. In particular, since sunlight is an ultimate energy source on which humankind depends, light energy conversion by photosynthesis and carbon dioxide (CO2 fixation) fixation should be improved and developed further. The improved CO2 fixation, which may use less adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), can be more efficiently applied to produce much more carbonaceous products.
Carbon dioxide fixation occurs in plants, algae and various microorganisms. Up to now, there are six carbon dioxide fixation cycles on Earth (Berg. 2011. Appl Environ Mirobiol. 77: 1925-1936). A cycle referred to as a Calvin cycle in which rubisco enzyme (ribulose-1,5-bisphosphate carboxylase/oxygenase) works is best known among these carbon dioxide fixation cycles. Calvin cycle is present in living bodies in which oxygenic photosynthesis takes place, including plants, algae, and cyanobacteria, and are also widely distributed in purple non-sulfur bacteria and non-photosynthetic bacteria, in which anoxygenic photosynthesis takes place. In Calvin cycle three carbon dioxide molecules are fixed to synthesize one glyceraldehyde-3-phosphate. This process requires eight ATP molecules to fix one carbon dioxide molecule, the amount of which is calculated based on 2.5 ATP yield from one NADPH. Rubisco has a very low enzymatic turnover number (Kcat) of 1 to 12 s−1. This problem is overcome by increasing an expression level of rubisco in plants so that the enzyme accounts for approximately 50% of water-soluble proteins in chloroplasts. Therefore, efforts have been made to improve kinetic parameters of rubisco in order to enhance light energy utilization efficiency, but did not yet come to fruition.
Reductive citric acid cycle is a process that proceeds in an opposite direction of a citric acid cycle and in which two carbon dioxide molecules are fixed to synthesize one acetyl coenzyme A (acetyl-CoA). This pathway was first found in green sulfur bacteria including Chlorobium tepidum. In this carbon dioxide fixation cycle, a 2-oxoglutarate synthase and isocitrate dehydrogenase have an ability to fix carbon dioxide, and they consume 5.5 ATP molecules (the amount of ATP is calculated as described above) to fix one carbon dioxide molecule.
Reductive acetyl-CoA cycle is also referred to as a Wood-Ljungdahl pathway, and was first found in Clostridia sp., that is, Moorella thermoacetica. This carbon dioxide fixation cycle fixes two carbon dioxide molecules to synthesize acetyl-CoA. In this process, six ATP molecules (the amount of ATP is calculated as described above) are consumed to fix one carbon dioxide molecule.
A 3-hydroxypropionate cycle is present in Chloroflexus aurantiacus that is a green non-sulfur bacterium. This carbon dioxide fixation cycle fixes three carbon dioxide molecules to synthesize pyruvate. In this process, seven ATP molecules (the amount of ATP is calculated as described above) are consumed to fix one carbon dioxide molecule. A modified 3-hydroxypropionate cycle referred to as a 3-hydroxypropionate/4-hydroxybutyrate cycle is present in Archaea sp. such as Metallosphaera sedula, and another modified 3-hydroxypropionate cycle referred to as a dicarboxylate/4-hydroxybutyrate cycle is also found in Archaea sp. such as Ignicoccus hospitalis. These two carbon dioxide fixation cycles commonly fix two carbon dioxide molecules to synthesize acetyl-CoA. In both processes, seven ATP molecules (the amount of ATP is calculated as described above) are consumed to fix one carbon dioxide molecule.
The contents described as the background art are merely provided to help in understanding the background of the present invention, and thus it should not be taken as an admission that they correspond to the conventional art already known to those of ordinary skill in the related art.